Generally, a hydraulic-type injection molding machine has an aspect that it is difficult to precisely control a position and a speed compared to an electromotive-type injection molding machine. In other words, in the case of the hydraulic type, as it uses hydraulic oil and a hydraulic actuator, viscosity, volume, and the like of the hydraulic oil changes according to temperature, and an inertial force is generated in the hydraulic actuator. These physical behaviors directly affect the control precision and responsiveness. In the case where, as a hydraulic pump, a variable discharge type hydraulic pump capable of controlling a discharge flow rate by varying the number of revolutions of a drive motor is used, length of the hydraulic circuit from the hydraulic pump to the hydraulic actuator gets longer, thereby influence of changes in viscosity, volume, and the like of the hydraulic oil being larger. When supposing, as a hydraulic actuator, for example, a mold clamping cylinder equipped in a mold clamping device, even though feedback control is carried out for the positions, mold opening positions (mold opening stop positions) largely vary among shots, causing unnecessary overrun of the movable mold, furthermore a trouble such as breakage and damage from colliding a molded product taking-out device with a mold (three-plate mold). This problem is more serious, when a mold opening time period is accelerated to shorten a molding cycle time in order to increase productivity.
Meanwhile, hitherto, a control method which aims at obtaining a precise mold opening position in a mold clamping device is known. JP 09-222924 A (Patent literature 1) discloses a method for controlling a mold-opening completion position in a direct pressure-type mold clamping device in which, with allowing a mold open process, eventually a molding cycle, to be shortened, variation in stop positions at the times of mold opening completion of a movable platen is corrected thereby to reduce the variation range, and suppress and prevent product mischucking by a taking-out machine. This method for controlling a mold-opening completion position works as follows: in a direct pressure-type mold clamping device, when the mold opening speed is switched from a high-speed mold opening section to a low-speed mold opening section to complete the mold open process, the high-speed mold opening section is longer while the low-speed mold opening section is as short as possible, thereby shortening the mold open process; and a distance of the stop position of the movable platen from its reference position at the time of the mold opening completion is sampled successively plural times as the mold opening operation is repeated and, when the variation range of the sampled distances exceeds the permissible range of the target stop position precision of the movable platen, the switching position from the high-speed mold opening section to the low-speed mold opening section is corrected to hold the variation range within the permissible range.